The invention relates to an online monitoring and diagnostic equipment for the initial failure of a synchronizing gear box. The invention adopts the technical proposal that: 2N (N is a positive integer) acceleration sensors are arranged at the positions to be tested of the synchronizing gear box; the output ends of the acceleration sensors are connected with the input ends X0 to XN-1 of first and second N-way multiway switches; the output public terminal COM of the first N-way multiway switch is connected with input ends IN+ with the same number of two subsystems, and the output public terminal COM of the second N-way multiway switch is connected with input ends IN- with the reverse number of the two subsystems. The output ends AH0 to AH2 of the two subsystems are connected with the input ends BH0 to BH5 of an A/D transition card, and the output ends D0 to D2n-1 of the A/D transition card are connected with a PCI expansion slot of a master control machine which is connected with a server by Ethernet. The expert system fault analysis software is installed in the master control machine and operated on a Labview operating platform. The equipment of the invention is characterized by convenient operation, and reflecting the initial failure of the synchronizing gear box promptly and accurately.